Epithelial Oncogenic KRAS Drives Immunosuppression in Pancreatic Cancer

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer. PDAC is one of the deadliest malignancies with a poor 5-year survival rate of 11%. The high metastatic capability and its late detection are among the reasons why it is so lethal. The best treatment is surgical resection but unfortunately most patients have unresectable tumors or metastatic disease. The standard of care for metastatic PDAC is treatment with Gemcitabine or for patients with more tolerance is FOLFIRINOX. However, these chemotherapies have a moderate improvement in overall survival and unfortunately most patients develop resistance. Therefore, it is critical to have a better understanding of this disease to discover potential therapeutic avenues. KRASG12D is the main mutation found in over 90% of human pancreatic tumor samples. One of the hallmarks of PDAC is the extensive fibroinflammatory response. Most of the tumor volume in humans is composed by stroma, particularly fibroblasts and immune cells. Despite, the abundant infiltration of immune cells, PDAC tumors are known as cold tumors because most of the cells are immunosuppressive. These immunosuppressive cells [tumor associated macrophages (TAMs), myeloid derived suppressor cells (MDSCs) or regulatory T cells (T regs)] interrupt T cell anti-tumor immunity. In fact, CD8+ T cells are very scarce in murine and human PDAC tumors. Most of the efforts now are trying to understand the immunosuppressive environment and how to reverse it. In our lab, we have published abundantly about different immunosuppressive mechanisms involving the crosstalk of multiple cell types. We have mainly used a multi-modal approach by using a combination of methods: genetically engineered mouse models, immunohistochemistry, flow cytometry, CyTOF, single cell RNA sequencing and bulk RNA sequencing to dissect the crosstalk between immune cells, fibroblasts, and tumor cells. In my dissertation, I first described in chapter 2, the initial events in the establishment of the precursor lesion microenvironment (PME) using a mouse model in which epithelial-oncogenic Kras can be activated or inactivated at will. We found that oncogenic KRAS drives the reprogramming of fibroblasts early during pancreatic carcinogenesis to an inflammatory phenotype. The cytokines produced by fibroblasts (IL6, IL33, CXCL1 and SAA3) have receptors in myeloid cells, including macrophages. Both, fibroblasts, and macrophages activate the JAK/STAT3 pathway which is downregulated upon oncogenic Kras inactivation. Furthermore, macrophages in the PME are modulated by oncogenic Kras by having an immunosuppressive phenotype when KRAS is activated and a tumor repair phenotype upon KRAS inactivation. Secondly, I described a project that focused in understanding the mechanism by which myeloid cells are immunosuppressive. We specifically targeted the JAK/STAT3 pathway in the myeloid compartment and used a syngeneic tumor transplantation model to study the effect of deleting myeloid-STAT3 in pancreatic cancer progression. We found that STAT3 was a major player in driving immunosuppression by myeloid cells and deleting it from myeloid cells decrease immunosuppressive cells and increase the number of T cells. All together my data show that oncogenic KRAS drives immunosuppression early during carcinogenesis and identifies the JAK/STAT3 pathway as a main pathway driving immunosuppression by myeloid cells.